Baghdad: An onslaught of bombings and shootings killed 113 people and wounded 250 across Iraq on Monday, officials said, in the nation's deadliest day so far this year.

The attacks come days after the leader of al-Qaeda in Iraq declared a new offensive and warned in a statement that the militant group is reorganising in areas from which it retreated before US troops left the country last December.

Al-Qaeda has been seeking to re-assert its might in the security vacuum left by the departing Americans, seizing on Baghdad's fragmented government and the surge of Sunni rebels in neighbouring Syria to sow instability across Iraq.

The security was tightened on Tuesday in Baghdad neighbourhoods.

Overall, 29 separate attacks were launched in 19 cities, shattering the relative calm that had held in the lead-up to the start of the holy Muslim fasting month of Ramadan.

Extra police and soldiers as well as security force vehicles were deployed today at marketplaces in neighbourhoods of Baghdad hit by the violence, witnesses said.

In the deadliest incidents -- a string of roadside bombs and a car bomb followed by a suicide attack targeting emergency responders in the town of Taji, just north of Baghdad -- at least 42 people were killed and 40 wounded, medical officials said.

"I was sleeping, and my parents lifted me out of bed," recalled 11-year-old Mohammed, whose family's home in Taji was levelled by the blast.

"They were shouting, 'Get up quickly! Help your younger brothers! There is a car bomb!'"

"It felt like it was just a few moments later when it exploded, and there was a big ball of fire, and the wreckage was falling over our heads."